1. Field of the Invention
The present invention generally relates to systems for controlling continuous sheetmaking systems and, more specifically, to sensors for measuring the fiber weight of wetstock in a papermaking machine.
2. State of the Art
In the alt of modem high-speed papermaking, it is well known to continuously measure certain properties of the paper material in order to monitor the quality of the finished product. These on-line measurements often include basis weight, moisture content, and sheet caliper (i.e., thickness). The measurements can be used for controlling process variables with the goal of maintaining output quality and minimizing the quantity of product that must be rejected due to upsets in the manufacturing process.
The on-line sheet property measurements are often accomplished by scanning sensors that periodically traverse the sheet material from edge to edge. For example, a high-speed scanning sensor may complete a scan in a period as short as twenty seconds, with measurements being read from the sensor at about 50 milliseconds intervals. It is also know that a series of stationary sensors can be used to make similar on-line measurements.
In the manufacture of paper on continuous papermaking machines, a web of paper is formed from an aqueous suspension of fibers (stock) on a traveling mesh papermaking fabric and water drains by gravity and suction through the fabric. The web is then transferred to the pressing section where more water is removed by pressure and vacuum. The web next enters the dryer section where steam heated dryers and hot air completes the drying process. The paper machine is, in essence, a de-watering, i.e., water removal, system. A typical forming section of a papermaking machine includes an endless traveling papermaking mesh fabric or wire which travels over a series of water removal elements such as table rolls, foils, vacuum foils, and suction boxes. As the material travels on the mesh fabric over the series of water removal elements, there is a distinct line of demarcation showing a change in the state of the stock from an extremely wet state to a relatively dryer state. This visible line of demarcation (referred to as the dry line) is characterized in that one side of the dry line has a glossy appearance (i.e. wet state) and the other side of the line has a non-glossy appearance (i.e., relatively dry state). The stock is carried on the top surface of the papermaking fabric and is de-watered as the stock travels over the successive de-watering elements to form a sheet of paper. Finally, the wet sheet is transferred to the press section of the papermaking machine where enough water is removed to form a sheet of paper. Other papermaking devices well known in the art are described for example in U.S. Pat. No. 5,400,258.
In another type of papermaking system (referred to as a twin wire machine), two meshes are used. A first mesh resides on the top of the stock and a second mesh resides underneath and supports the stock. Water is removed from the top by vacuum and from the bottom by gravity (and in some cases also by vacuum). The advantage to this type of system is that water is removed at a much quicker rate than in the previously described single mesh system, resulting in a faster machine speed, a more controllable process, and more uniform paper product. In addition, this system is smaller since it requires less mesh length, has a shorter drying time, and consequently, reduced processing costs.
Many factors influence the rate at which water is removed which ultimately affects the quality of the paper produced. As is apparent. it would be advantageous to monitor the dynamic process so as to, among other things, predict and control the dry stock weight of the paper that is produced.
It is conventional to measure the moisture content on leaving the main dryer section or at the take up reel employing scanning sensors. Such measurement may be used to adjust the machine operation toward achieving desired parameters. One technique for measuring moisture content is to utilize the absorption spectrum of water in the infra-red. Monitoring or gauge apparatus for this purpose is commonly in use. Such apparatus conventionally uses either a fixed gauge or a gauge mounted on a scanning head which is repetitively scanned transversely across the web at the exit from the dryer section and/or upon entry to the take up reel, as required by the individual machines. The gauges typically use a broad-band infra-red source and one or more detectors with the wavelength of interest being selected by a narrow-band filter, for example, an interference type filter. The gauges used fall into two main types: the transmissive type in which the source and detector are on opposite sides of the web and, in a scanning gauge, are scanned in synchronism across it, and the scatter type (sometimes called "reflective" type) in which the source and detector are in a single head on one side of the web, the detector responding to the amount of source radiation scattered from the web.